It is common to mount a reciprocating engine with resilient mounting assemblies to isolate the frame from engine vibration. Another not often mentioned benefit of resilient mounting is the accommodation of manufacturing tolerances when mating two relatively rigid assemblies such as an engine and automobile frame. Furthermore, resilient mounting accommodates flexure of the frame caused by the engine working torque and vehicle dynamics. Vehicle dynamics includes stresses and strains caused by movement over uneven road surfaces, acceleration forces to increase velocity, braking forces to slow the vehicle, and forces generated when going around corners.
E. B. Etchells in U.S. Pat. No. 2,953,336 teaches the common three point resilient mounting of an engine transmission assembly into an automobile frame. This patent includes discussion of the nodal positioning of the engine mounts to minimize vibrations while controlling engine torque and accommodating road induced vibrations. This system incorporates a single resilient mounting at the rear of the engine assembly and a pair of transversely spaced resilient mounts at the front of the engine. The nodal point is a place of minimum vibration. Positioning of the front engine mounts as close as is practical to the percussion points of the engine assembly reduces road induced loads on the rear mount and allows the rear mount to be soft and compliant.
The mounting system of Etchells is widely utilized and there exist improvement patents such as Fehlberg, U.S. Pat. No. 3,731,896, that demonstrates continued applicability. Fehlberg teaches the need for mechanical limits to retain the engine transmission assembly to the frame when the strength limits of resilient elastic elements are exceeded.
R. E. Krueger, in U.S. Pat. No. 3,146,986, discusses the need for torque measurement in automobiles, boats and small airplanes. The embodiment shown includes hydraulic sensing means for measuring torque, and is mounted parallel to a resilient elastic engine mount in an automobile.
The engine in an automobile is heavy, generates significant torque and must be firmly attached to the frame to resist road dynamics. These considerations require that the resilient elastic mount be of sufficient stiffness to prohibit excessive engine movements. Mounting a sensor in parallel to the resilient mount induces measurement error caused by frame deflection, thermal expansion or contraction of the elastic element and temperature induced elastic stiffness changes. The zero adjusting unit provided in the Krueger apparatus can only be effective if all conditions are static after adjustment and during the time measurements are taken. Repeatability and accuracy are affected when measurements are taken in parallel to the engine retention components of the engine mount.
G. L. Malchow, in U.S. Pat. No. 3,903,738, discloses a torque-sensing device that replaces one of the engine mounts in an engine installation as depicted in Etchells. Malchow removes one of the resilient mounts and replaces it with a strain gage-equipped pivotal yoke assembly. In this configuration, the engine is restrained from rotational movement by a force couple applied on one side by the elastic engine mount and on the other side by the strain gage-equipped pivotal yoke assembly. The configuration of the yoke assembly of the strain gage equipped engine mount makes determination of the length of moment arm and the magnitude of restraining force a complex geometrical problem. Malchow avoids these issues by calibrating the apparatus “where weights were suspended from a torque arm which was connected to the transmission out put shaft.”
The stability of the complex geometry that determines torque arm length affects calibration and repeatability of the torque measurement. The location of the restraining force through the resilient elastic mount is subject to movement-induced creep or sag. Resilient elastic supports undergo creep and sag over time due to thermal and long term loading. Also, frame flexure due to road induced loads can cause lateral displacements between the frame mounting points of the front engine mounts, changing the inclination of the yoke, and significantly altering the calibration of torque measurement.
The yoke assembly does not restrain the engine from movement due to acceleration loads caused by braking or acceleration. These loads are restrained by the resilient engine mount on the side opposite the yoke assembly and the compliant mount on the transmission. Aside from potential safety issues, the resilient engine mounts will allow movement that may result in damage to the yoke assembly and/or inaccurate torque measurement.
A three point mounting system, with a sensor at one of the mounting points, has an effective pivotal axis through the other two mounting points. The center of gravity of the engine mass is significantly displaced both vertically and laterally from the pivotal axis of the engine, thereby departing from the teachings of Etchells regarding the importance of nodal positioning of the mounts.
Even when vehicle velocity and engine torque are constant, the lateral or sideways displacement of the center of gravity with respect to the pivotal axis allows vertical accelerations of the vehicle, such as those caused by movement while traveling over bumps in the road, to create forces that result in false torque measurements.
Similarly, even when vehicle velocity and engine torque are constant, vertical displacement of the center of gravity from the pivotal axis allows cornering accelerations caused by the vehicle going around turns to create forces that result in false torque measurements.
Also, even if engine torque is constant, combined vertical and lateral displacement of the center of gravity from the pivotal axis along with an inclined pivotal axis allows longitudinal accelerations resulting in vehicle velocity changes to create forces that result in false torque measurements.